1. Field of the Invention
The present invention relates to physically-chemically and pharmacokinetically enhanced Nω-hydroxy-L-arginine (NOHA) derivatives and a method for producing the NOHA derivatives having enhanced physical-chemical and pharmacokinetic properties according to the invention.
2. Discussion of the Prior Art
Nω-hydroxy-L-arginine is the physiologically occurring intermediate of the NO synthase catalysed oxidation L-arginine. NOHA is oxidised in a further step of NO synthase, nitrogen monoxide being released and L-citrulline being formed.
Thus the semi-essential amino acid L-arginine is the natural source for the nitrogen monoxide (NO).
Nitrogen monoxide (NO) is of decisive importance among others for supplying the organs with blood. NO leads indirectly to an increase of the vessels. The extent of the vascular dilation then has different effects in the individual organs. In the heart there is for example an improved circulation. In addition to expanding the vessels, NO also has other properties:                The relaxing effect concerns not only the musculature of the vessels but also that of the bronchial tree.        NO that is given off by the endothelial cells into the vessel lumen can prevent the accumulation of blood platelets (=thrombus formation).        In the nervous system, it is an important signalling substance (transmitter) that influences the brain and gastrointestinal functions. Thus nerve ends situated in the intestinal wall cause a relaxation of the ring muscle due to the release of NO.        NO is formed in defence cells (macrophages) and is capable of destroying bacteria. Stimulated by bacterial components (e.g. lipopolysaccharides) macrophages produce NO in high concentrations so that vital enzymes, e.g. those containing iron, are blocked. The conservation of meat for example by salting is based precisely on this process.        
A dysregulation or reduced NO availability is thus related to diverse cardiovascular diseases. A limited NO availability is therefore associated with the so-called endothelial dysfunction—a state of multifactorial genesis—that is associated with high blood pressure, atherosclerosis, arterial thrombosis, coronary heart disease, heart failure, heart attack, hypercholesterolaemia and diabetes.
Old, rigid, atherosclerotically changed vessels can be deformed again with NO. Blood circulation is thus improved and among others high blood pressure can be normalised. For children that are born with serious respiratory dysfunction, the inhalation of NO is even today in use successfully. NO promotes the erection of the penis which has led to the development of drugs against impotence (Viagra®).
Also for fighting tumors it is expected that new treatments can be pursued with NO since NO produced by white blood cells does not only destroy bacteria but also cells.
However, NO has quite a short life. Within a short period it reacts with oxygen molecules to form nitrite (NO2−) and nitrate (NO3−). The short life also explains why NO can only be formed directly at its target.
It is further known that NOHA is a potent inhibitor of arginase I with a Ki value between 30-42 μM.
In smaller in-vivo studies with rats, desired effects for treating the erectile dysfunction, endothelial dysfunction and hypertension could already be demonstrated using an i.v.-NOHA therapy.
Treating diseases associated with endothelial dysfunction, using conventional NO donors has a few disadvantages. In the case of nitrates for example the short therapeutic half time, the low oral bioavailability, partly adverse haemodynamic effects and tolerance effects are to be mentioned in this context. Indeed, a proatherogene effect during long-term treatment using organic nitrates is being discussed recently.
So that a therapy of NO-deficient diseases is possible with few side effects, the physiological situation has to be imitated as best as possible, i.e. NO must only be released for a short time, in the right amounts and of the right location. All these prerequisites are met using NOHA as an NO-donor or, to take into consideration the time factor, with a retarding prodrug of NOHA. In addition to the fact that using this strategy, nitrogen monoxide is only released where it is needed and is formed to an extent that is too small, the fact can be exploited that NOHA presents one of the most potent arginase inhibitors. Specifically an increased arginase activity is discussed as a mechanism for reduced NO availability and thus as a contributing factor in the development of the endothelial dysfunction. Thus a dual mode of action is exploited when using NOHA.
The use of L-arginine and Nω-hydroxy-L-arginine and their simple carboxylic acid esters, and also analogue N-hydroxyguanidine for treating a multiplicity of diseases, have been described and patented (a selection: WO03045369, U.S. Pat. No. 6,277,884, WO0132167, CA02386938).
In practice, use of such compounds is limited by their bad pharmacokinetic profile. The insufficient drug qualities of this substance can be explained above all by the presence of an unsubstituted N-hydroxyguanidine function. In particular, the following effects can be observed for the physical-chemical instability of N-hydroxyguanidines:
N-hydroxyguanidines decompose at room temperature and should be stored at 4-8° C., better −20° C. They are most stable in the form of their salts of strong acids. The following decomposition processes are known:                (1) Hydrolysis sensitivity: In particular at higher pH (>pH 7) conversion takes place to cyanamides and hydrolysis to ureas, which is relevant when taking into account the physiological pH of 7.4 in vivo.        (2) Oxidation sensitivity: The susceptibility to oxidation is probably the biggest issue of this class of substances since using many and very different oxidation agents it could be shown that two- and three-electron oxidation is possible. Using singly substituted aliphatic and aromatic hydroxyguanidines, oxidation potentials of Eox1=+0.51-0.62 V and Eox2=+1.14-1.81 V could be determined. The decomposition products can differ as a function of the oxidation agent. Also metal cations as for example iron(II)(III) or copper(II) favour such processes or participate in them. These processes are physiologically relevant since it is known that oxidative processes dominate in vivo and quite a range of physiological substances (reactive oxygen species, metal cations) favour these oxidation processes.        
Metabolic instability of N-hydroxyguanidines:                N-hydroxyguanidines are effectively reduced in the body in an enzymatic catalysed manner to form corresponding guanidines. The enzyme systems responsible for this have already been partly identified. Thus the mitochondrial N-reduction is dependent on cytochrome b5, cytochrome b5 reductase and a molybdenum cofactor-dependent enzyme (mARC). The microsomale N-reduction is catalysed by cytochrome b5, cytochrome b5-reductase and a third component unknown so far.        In addition it is known that hydroxyguanidines can be O-glucuronidised within the framework of a metabolic phase II biotransformation to be converted into a form that can be excreted more easily.        
This leads to a strong limitation of the biological half time as a result of thermal, hydrolytic, oxidative and enzymatic processes. Also a high first-pass effect is to be expected, considering the metabolic instability that was mentioned.